In case of Full-Bore meters, the variation of flow rate through a constant area is related to the flow rate. Unlike full-Bore meters, area flow meters are devices in which pressure drop is constant and the area through which the fluid flows varies with flow rate.

Rotameter

Rotameter is a typical area meter. It consists of gradually tapered glass mounted vertically in a frame with the large end up. Fluid flows upward through the tapered tube and suspends freely a float (which is submerged in the fluid). Float is the indicating element, and the greater the flow rate, the higher the float rides in the tube.
 
The tube is marked in divisions, and the reading of the meter is obtained from the scale reading at the reading edge of the float, which is taken at the largest cross section of the float.

A calibration curve must be available to convert the observed scale reading to flow rate.

Advantages of a Rotameter

Can be modified to cater to the needs of individual operation. For example, in case of a rotameter with a metal case, the float can have an extension.

The float of the rotameter rotates to indicate smooth operation. Float shapes and proportions can be varied for different applications.

Flow rate through a Rotameter

where q - Volumetric flow rate umax - Maximum Velocity of the fluid Dt - Tube Diameter Df - Float Diameter